Despite the large numbers of effective antibiotics in use today, the 'post-antimicrobial era'has been looming for well over a decade as patients, particularly in intensive care units (ICUs), are increasingly developing infections caused by multidrug-resistant (MDR) pathogens. Such infections are associated with increased mortality, increased morbidity, and increased cost. Gram negative bacilli (GNB), such as Pseudomonas aeruginosa, Klebsiella pneumonia and Acinetobacter spp. (the pathogens of interest in this project), may become resistant to all antimicrobial agents as determined by routine susceptibility testing. The goals of this project are to: 1) determine strategies to improve the treatment of patients infected with MDR-GNB and thus improve clinical and microbiological outcomes of such infections, (2) assess which laboratory algorithms for in vitro antimicrobial susceptibility testing most closely correlate with improved outcomes for MDR-GNB infections, and (3) evaluate how healthcare professionals actually use in vitro susceptibility data to choose and modify treatment regimens for MDR-GNB infections. The research design is a prospective observational study of ICU patients with hospital-acquired pneumonia and bloodstream infections caused by MDR-GNB at New York- Presbyterian Hospital (NYPH) in New York City, the epicenter of MDR-GNB infections. NYPH is the largest medical center in NYC and has 16 ICUs, 383 ICU beds, and 14,800 patients admitted to the ICU each year of whom approximately 40-50 develop MDR-GNB infections. The specific aims and research methods are as follows: Aim 1 will observe the treatment strategies used for MDR-GNB infections and determine the outcomes of subjects with MDR-GNB infections associated with selected treatment regimens and as compared with the outcomes of subjects with non-MDR-GNB infections by performing a nested case-control study. Aim 2 will determine the in vitro susceptibility testing strategies (performed on MDR-GNB from Aim 1) that best predict successful clinical and microbiologic outcomes and potentially lead to different treatment choices. Testing strategies will include ancillary testing currently used by NYPH clinical microbiology laboratories (e.g., Etest for colistin and tigecycline) as well as research assays (e.g., reference broth microdilution synergy studies, Etest synergy studies, and minimal bactericidal concentration testing). Aim 3 will survey healthcare professionals at NYPH to determine their knowledge, attitudes, and practices regarding antimicrobial susceptibility testing strategies and determine factors associated with the use of test results. To do so, an electronic, anonymous survey will be administered to 150 ICU physicians, infectious diseases sub-specialists, and clinical pharmacists. The carefully conducted observational studies proposed in this project, supplemented by potentially clinically relevant in vitro susceptibility testing, hold the greatest promise to improve the treatment of patients infected with MDR-GNB. Patients, particularly in intensive care units, are increasingly developing infections caused by multidrug-resistant organisms. These infections are associated with increased mortality, increased morbidity, and increased cost. Carefully conducted observational studies and clinically relevant in vitro antimicrobial susceptibility testing, hold the greatest promise to improve the treatment of patients infected with mutidrug-resistant organisms.